Electrically and mechanically tunable electron spins in silicon carbide color centers

Transition-Metal-Related Quantum Emitters in Wurtzite AlN and GaN

Transition-metal centers exhibit a paramagnetic ground state in wide-bandgap semiconductors and are promising for nanophotonics and quantum information processing. Specifically, there is a growing interest in discovering prominent paramagnetic spin defects that can be manipulated using optical methods. Here, we investigate the electronic structure and magneto-optical properties of Cr and Mn substitutional centers in wurtzite AlN and GaN. We use state-of-the-art hybrid density functional theory calculations to determine level structure, stability, optical signatures, and magnetic properties of these centers. The excitation energies are calculated using the constrained occupation approach and rigorously verified with the complete active space configuration interaction approach. Our simulations of the photoluminescence spectra indicate that Cr Al 1 + in AlN and Cr Ga 1 + in GaN are responsible for the observed narrow quantum emission near 1.2 eV. We compute the zero-field splitting (ZFS) parameters and outline an optical spin polarization protocol for Cr Al 1 + and Cr Ga 1 + . Our results demonstrate that these centers are promising candidates for spin qubits.

7D High-Dynamic Spin-Multiplexing

Multiplexing technology creates several orthogonal data channels and dimensions for high-density information encoding and is irreplaceable in large-capacity information storage, and communication, etc. The multiplexing dimensions are constructed by light attributes and spatial dimensions. However, limited by the degree of freedom of interaction between light and material structure parameters, the multiplexing dimension exploitation method is still confused. Herein, a 7D Spin-multiplexing technique is proposed. Spin structures with four independent attributes (color center type, spin axis, spatial distribution, and dipole direction) are constructed as coding basic units. Based on the four independent spin physical effects, the corresponding photoluminescence wavelength, magnetic field, microwave, and polarization are created into four orthogonal multiplexing dimensions. Combined with the 3D of space, a 7D multiplexing method is established, which possesses the highest dimension number compared with 6 dimensions in the previous study. The basic spin unit is prepared by a self-developed laser-induced manufacturing process. The free state information of spin is read out by four physical quantities. Based on the multiple dimensions, the information is highly dynamically multiplexed to enhance information storage efficiency. Moreover, the high-dynamic in situ image encryption/marking is demonstrated. It implies a new paradigm for ultra-high-capacity storage and real-time encryption.

Quantum systems in silicon carbide for sensing applications

This paper summarizes recent studies identifying key qubit systems in silicon carbide (SiC) for quantum sensing of magnetic, electric fields, and temperature at the nano and microscale. The properties of colour centres in SiC, that can be used for quantum sensing, are reviewed with a focus on paramagnetic colour centres and their spin Hamiltonians describing Zeeman splitting, Stark effect, and hyperfine interactions. These properties are then mapped onto various methods for their initialization, control, and read-out. We then summarised methods used for a spin and charge state control in various colour centres in SiC. These properties and methods are then described in the context of quantum sensing applications in magnetometry, thermometry, and electrometry. Current state-of-the art sensitivities are compiled and approaches to enhance the sensitivity are proposed. The large variety of methods for control and read-out, combined with the ability to scale this material in integrated photonics chips operating in harsh environments, places SiC at the forefront of future quantum sensing technology based on semiconductors.

Accurate Measurement of Defect Generation Rates in Silicon Carbide Irradiated with Energetic Ions

In this work, we obtained the Si vacancy generation rates η in SiC nanowire samples irradiated with 1, 3 MeV protons, and 2.8 MeV helium ions using the electrical resistivity measurement, which further indicated an intuitive linear function correlation between η and the nuclear stopping power of the incident ions at a low dpa level with a coefficient of 2.15 × 10-3 eV-1. Prediction through this correlation is consistent with previous work. Besides, the measured value is about 1/2 of the simulation results with the popular SRIM code. Overall, our work provides a feasible way to get the generation rate of a certain irradiation-induced defect by electric measurements, and the correlation obtained is practically useful in various applications.

Fiber-coupled silicon carbide divacancy magnetometer and thermometer

Divacancy in silicon carbide has become an important solid-state system for quantum metrologies. To make it more beneficial for practical applications, we realize a fiber-coupled divacancy-based magnetometer and thermometer simultaneously. First, we realize an efficient coupling between the divacancy in a silicon carbide slice with a multimode fiber. Then the optimization of the power broadening in optically detected magnetic resonance (ODMR) of divacancy is performed to obtain a higher sensing sensitivity of 3.9 μT/Hz^1/2. We then use it to detect the strength of an external magnetic field. Finally, we use the Ramsey methods to realize a temperature sensing with a sensitivity of 163.2 mK/Hz^1/2. The experiments demonstrate that the compact fiber-coupled divacancy quantum sensor can be used for multiple practical quantum sensing.

High-sensitivity silicon carbide divacancy-based temperature sensing

Color centers in silicon carbide have become potentially versatile quantum sensors. Particularly, wide temperature-range temperature sensing has been realized in recent years. However, the sensitivity is limited due to the short dephasing time of the color centers. In this work, we developed a high-sensitivity silicon carbide divacancy-based thermometer using the thermal Carr-Purcell-Meiboom-Gill (TCPMG) method. First, the zero-field splitting D of the PL6 divacancy as a function of temperature was measured with a linear slope of -99.7 kHz K^-1. The coherence times of TCPMG pulses linearly increased with the pulse number and the longest coherence time was about 21 μs, which was ten times higher than . The corresponding temperature-sensing sensitivity was 13.4 mK Hz^-1/2, which was about 15 times higher than previous results. Finally, we monitored the laboratory temperature variations for 24 hours using the TCMPG pulse. The experiments pave the way for the application of silicon carbide-based high-sensitivity thermometers in the semiconductor industry, biology, and materials sciences.

Magnetic-field-dependent spin properties of divacancy defects in silicon carbide

In recent years, spin defects in silicon carbide have become promising platforms for quantum sensing, quantum information processing and quantum networks. It has been shown that their spin coherence times can be dramatically extended with an external axial magnetic field. However, little is known about the effect of magnetic-angle-dependent coherence time, which is an essential complement to defect spin properties. Here, we investigate the optically detected magnetic resonance (ODMR) spectra of divacancy spins in silicon carbide with a magnetic field orientation. The ODMR contrast decreases as the off-axis magnetic field strength increases. We then study the coherence times of divacancy spins in two different samples with magnetic field angles, and both of the coherence times decrease with the angle. The experiments pave the way for all-optical magnetic field sensing and quantum information processing.

Single-Spin Readout and Quantum Sensing Using Optomechanically Induced Transparency

Solid-state spin defects are promising quantum sensors for a large variety of sensing targets. Some of these defects couple appreciably to strain in the host material. We propose to use this strain coupling for mechanically mediated dispersive single-shot spin readout by an optomechanically induced transparency measurement. Surprisingly, the estimated measurement times for negatively charged silicon-vacancy defects in diamond are an order of magnitude shorter than those for single-shot optical fluorescence readout. Our scheme can also be used for general parameter-estimation metrology and offers a higher sensitivity than conventional schemes using continuous position detection.

An electrically driven single-atom "flip-flop" qubit

The spins of atoms and atom-like systems are among the most coherent objects in which to store quantum information. However, the need to address them using oscillating magnetic fields hinders their integration with quantum electronic devices. Here, we circumvent this hurdle by operating a single-atom "flip-flop" qubit in silicon, where quantum information is encoded in the electron-nuclear states of a phosphorus donor. The qubit is controlled using local electric fields at microwave frequencies, produced within a metal-oxide-semiconductor device. The electrical drive is mediated by the modulation of the electron-nuclear hyperfine coupling, a method that can be extended to many other atomic and molecular systems and to the hyperpolarization of nuclear spin ensembles. These results pave the way to the construction of solid-state quantum processors where dense arrays of atoms can be controlled using only local electric fields.

Field programmable spin arrays for scalable quantum repeaters

The large scale control over thousands of quantum emitters desired by quantum network technology is limited by the power consumption and cross-talk inherent in current microwave techniques. Here we propose a quantum repeater architecture based on densely-packed diamond color centers (CCs) in a programmable electrode array, with quantum gates driven by electric or strain fields. This 'field programmable spin array' (FPSA) enables high-speed spin control of individual CCs with low cross-talk and power dissipation. Integrated in a slow-light waveguide for efficient optical coupling, the FPSA serves as a quantum interface for optically-mediated entanglement. We evaluate the performance of the FPSA architecture in comparison to a routing-tree design and show an increased entanglement generation rate scaling into the thousand-qubit regime. Our results enable high fidelity control of dense quantum emitter arrays for scalable networking.

Coherent Control and Magnetic Detection of Divacancy Spins in Silicon Carbide at High Pressures

Spin defects in silicon carbide appear to be a promising tool for various quantum technologies, especially for quantum sensing. However, this technique has been used only at ambient pressure until now. Here, by combining this technique with diamond anvil cell, we systematically study the optical and spin properties of divacancy defects created at the surface of SiC at pressures up to 40 GPa. The zero-field-splitting of the divacancy spins increases linearly with pressure with a slope of 25.1 MHz/GPa, which is almost two-times larger than that of nitrogen-vacancy centers in diamond. The corresponding pressure sensing sensitivity is about 0.28 MPa/Hz^-1/2. The coherent control of divacancy demonstrates that coherence time decreases as pressure increases. Based on these, the pressure-induced magnetic phase transition of Nd₂Fe₁₄B sample at high pressures was detected. These experiments pave the way to use divacancy in quantum technologies such as pressure sensing and magnetic detection at high pressures.

Modeling and Simulations of 4H-SiC/6H-SiC/4H-SiC Single Quantum-Well Light Emitting Diode Using Diffusion Bonding Technique

In the last decade, silicon carbide (SiC) has emerged as a potential material for high-frequency electronics and optoelectronics applications that may require elevated temperature processing. SiC exists in more than 200 different crystallographic forms, referred to as polytypes. Based on their remarkable physical and electrical characteristics, such as better thermal and electrical conductivities, 3C-SiC, 4H-SiC, and 6H-SiC are considered as the most distinguished polytypes of SiC. In this article, physical device simulation of a light-emitting diode (LED) based on the unique structural configuration of 4H-SiC and 6H-SiC layers has been performed which corresponds to a novel material joining technique, called diffusion welding/bonding. The proposed single quantum well (SQW) edge-emitting SiC-based LED has been simulated using a commercially available semiconductor device simulator, SILVACO TCAD. Moreover, by varying different design parameters, the current-voltage characteristics, luminous power, and power spectral density have been calculated. Our proposed LED device exhibited promising results in terms of luminous power efficiency and external quantum efficiency (EQE). The device numerically achieved a luminous efficiency of 25% and EQE of 16.43%, which is at par performance for a SQW LED. The resultant LED structure can be customized by choosing appropriate materials of varying bandgaps to extract the light emission spectrum in the desired wavelength range. It is anticipated that the physical fabrication of our proposed LED by direct bonding of SiC-SiC wafers will pave the way for the future development of efficient and cost-effective SiC-based LEDs.

Atomic Defect Induced Saturable Absorption of Hexagonal Boron Nitride in Near Infrared Band for Ultrafast Lasing Applications

Defect-induced phenomena in 2D materials has received increasing interest among researchers due to the novel properties correlated with precise modification of materials. We performed a study of the nonlinear saturable absorption of the boron-atom-vacancy defective hexagonal boron nitride (h-BN) thin film at a wavelength of ~1 μm and its applications in ultrafast laser generation. The h-BN is with wide band gap of ~6 eV. Our investigation shows that the defective h-BN has a wide absorption band from visible to near infrared regimes. First-principle calculations based on density functional theory (DFT) indicate that optical property changes may be attributed to the boron-vacancy-related defects. The photoluminescence spectrum shows a strong emission peak at ~1.79 eV. The ultrafast Z-scan measurement shows saturable absorbance response has been detected for the defective h-BN with saturation intensity of ~1.03 GW/cm² and modulation depth of 1.1%. In addition, the defective h-BN has been applied as a new saturable absorber (SA) to generate laser pulses through the passively Q-switched mode-locking configuration. Based on a Nd:YAG waveguide platform, 8.7 GHz repetition rate and 55 ps pulse duration of the waveguide laser have been achieved. Our results suggest potential applications of defective h-BN for ultrafast lasing and integrated photonics.

First-Principles Predictions of Out-of-Plane Group IV and V Dimers as High-Symmetry, High-Spin Defects in Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) has been recently found to host a variety of quantum point defects, which are promising candidates as single-photon sources for solid-state quantum nanophotonic applications. Most recently, optically addressable spin qubits in h-BN have been the focus of intensive research due to their unique potential in quantum computation, communication, and sensing. However, the number of high-symmetry, high-spin defects that are desirable for developing spin qubits in h-BN is highly limited. Here, we combine density functional theory (DFT) and quantum embedding theories to show that out-of-plane X_NY_i dimer defects (X, Y = C, N, P, and Si) form a new class of stable C_3v spin-triplet defects in h-BN. We find that the dimer defects have a robust ³A₂ ground state and ³E excited state, both of which are isolated from the h-BN bulk states. We show that ¹E and ¹A shelving states exist and they are positioned between the ³E and ³A₂ states for all the dimer defects considered in this study. To support future experimental identification of the X_NY_i dimer defects, we provide extensive characterization of the defects in terms of their spin and optical properties. We predict that the zero-phonon line of the spin-triplet X_NY_i defects lies in the visible range (800 nm to 500 nm). We compute the zero-field splitting of the dimers' spin to range from 1.79 GHz (Si_NP_i⁰) to 29.5 GHz (C_NN_i⁰). Our results broaden the scope of high-spin defect candidates that would be useful for the development of spin-based solid-state quantum technologies in two-dimensional hexagonal boron nitride.

Point Defects in Monolayer h-AlN as Candidates for Single-Photon Emission

A single-photon emission (SPE) system based on a solid state is one of the fundamental branches in quantum information and communication technologies. The traditional bulk semiconductors suffered limitations of difficult photon extraction and long radiative lifetime. Two-dimensional (2D) semiconductors with an entire open structure and low dielectric screening can overcome these shortcomings. In this work, we focus on monolayer h-AlN due to its wide band gap and the successful achievement of SPE compared to its bulk counterpart. We systematically investigate the properties of point defects, including vacancies, antisites, and impurities, in monolayer h-AlN by employing hybrid density functional theory calculations. The -1 charged Al vacancy (V_Al^-) and +1 charged nitrogen antisite (N_Al⁺) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex C_AlV_N⁺, which is composed of vacancies and carbon substitutions, also can be used for SPE. Our results extend the avenue for realizing SPE in 2D semiconductors.

Room temperature coherent manipulation of single-spin qubits in silicon carbide with a high readout contrast

Spin defects in silicon carbide (SiC) with mature wafer-scale fabrication and micro/nano-processing technologies have recently drawn considerable attention. Although room-temperature single-spin manipulation of colour centres in SiC has been demonstrated, the typically detected contrast is less than 2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\%$\end{document}, and the photon count rate is also low. Here, we present the coherent manipulation of single divacancy spins in 4H-SiC with a high readout contrast (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$-30\%$\end{document}) and a high photon count rate (150 kilo counts per second) under ambient conditions, which are competitive with the nitrogen-vacancy centres in diamond. Coupling between a single defect spin and a nearby nuclear spin is also observed. We further provide a theoretical explanation for the high readout contrast by analysing the defect levels and decay paths. Since the high readout contrast is of utmost importance in many applications of quantum technologies, this work might open a new territory for SiC-based quantum devices with many advanced properties of the host material.

Quantum Information and Algorithms for Correlated Quantum Matter

Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from correlations. It is the promise of quantum materials for quantum information science superimposed with the potential of new computational quantum algorithms to discover new quantum materials that inspires this Review. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, and computing. Simultaneously, efforts toward developing new quantum algorithmic approaches for quantum simulation and advanced calculation methods for many-body quantum systems enable major advances toward functional quantum materials and their deployment. The advent of quantum computing brings new possibilities for eliminating the exponential complexity that has stymied simulation of correlated quantum systems on high-performance classical computers. Here, we review new algorithms and computational approaches to predict and understand the behavior of correlated quantum matter. The strongly interdisciplinary nature of the topics covered necessitates a common language to integrate ideas from these fields. We aim to provide this common language while weaving together fields across electronic structure theory, quantum electrodynamics, algorithm design, and open quantum systems. Our Review is timely in presenting the state-of-the-art in the field toward algorithms with nonexponential complexity for correlated quantum matter with applications in grand-challenge problems. Looking to the future, at the intersection of quantum information science and algorithms for correlated quantum matter, we envision seminal advances in predicting many-body quantum states and describing excitonic quantum matter and large-scale entangled states, a better understanding of high-temperature superconductivity, and quantifying open quantum system dynamics.

Nanomagnonic Cavities for Strong Spin-Magnon Coupling and Magnon-Mediated Spin-Spin Interactions

We present a theoretical approach to use ferromagnetic or ferrimagnetic nanoparticles as microwave nanomagnonic cavities to concentrate microwave magnetic fields into deeply subwavelength volumes ∼10^{-13}  mm^{3}. We show that the field in such nanocavities can efficiently couple to isolated spin emitters (spin qubits) positioned close to the nanoparticle surface reaching the single magnon-spin strong-coupling regime and mediate efficient long-range quantum state transfers between isolated spin emitters. Nanomagnonic cavities thus pave the way toward magnon-based quantum networks and magnon-mediated quantum gates.

Strain Modulation of Si Vacancy Emission from SiC Micro- and Nanoparticles

Single-photon emitting point defects in semiconductors have emerged as strong candidates for future quantum technology devices. In the present work, we exploit crystalline particles to investigate relevant defect localizations, emission shifting, and waveguiding. Specifically, emission from 6H-SiC micro- and nanoparticles ranging from 100 nm to 5 μm in size is collected using cathodoluminescence (CL), and we monitor signals attributed to the Si vacancy (VSi) as a function of its location. Clear shifts in the emission wavelength are found for emitters localized in the particle center and at the edges. By comparing spatial CL maps with strain analysis carried out in transmission electron microscopy, we attribute the emission shifts to compressive strain of 2-3% along the particle a-direction. Thus, embedding VSi qubit defects within SiC nanoparticles offers an interesting and versatile opportunity to tune single-photon emission energies while simultaneously ensuring ease of addressability via a self-assembled SiC nanoparticle matrix.

Dynamic Polarization of Electron Spins Interacting with Nuclei in Semiconductor Nanostructures

We suggest a new spin orientation mechanism for localized electrons: dynamic electron spin polarization provided by nuclear spin fluctuations. The detrimental effect of nuclear spin fluctuations can be harnessed and employed to provide angular momentum for the electrons via the hyperfine interaction in a weak magnetic field. For this, the sample is illuminated by an unpolarized light, which directly polarizes neither the electrons nor the nuclei. We predict that, for the electrons bound in localized excitons, 100% spin polarization can be reached in longitudinal magnetic fields of a few millitesla. The proof of principle experiment is performed on momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic field of 17 mT the electron spin polarization of 30% is measured.

Anisotropic Spin-Acoustic Resonance in Silicon Carbide at Room Temperature

We report on acoustically driven spin resonances in atomic-scale centers in silicon carbide at room temperature. Specifically, we use a surface acoustic wave cavity to selectively address spin transitions with magnetic quantum number differences of ±1 and ±2 in the absence of external microwave electromagnetic fields. These spin-acoustic resonances reveal a nontrivial dependence on the static magnetic field orientation, which is attributed to the intrinsic symmetry of the acoustic fields combined with the peculiar properties of a half-integer spin system. We develop a microscopic model of the spin-acoustic interaction, which describes our experimental data without fitting parameters. Furthermore, we predict that traveling surface waves lead to a chiral spin-acoustic resonance that changes upon magnetic field inversion. These results establish silicon carbide as a highly promising hybrid platform for on-chip spin-optomechanical quantum control enabling engineered interactions at room temperature.

Universal coherence protection in a solid-state spin qubit

Decoherence limits the physical realization of qubits, and its mitigation is critical for the development of quantum science and technology. We construct a robust qubit embedded in a decoherence-protected subspace, obtained by applying microwave dressing to a clock transition of the ground-state electron spin of a silicon carbide divacancy defect. The qubit is universally protected from magnetic, electric, and temperature fluctuations, which account for nearly all relevant decoherence channels in the solid state. This culminates in an increase of the qubit's inhomogeneous dephasing time by more than four orders of magnitude (to >22 milliseconds), while its Hahn-echo coherence time approaches 64 milliseconds. Requiring few key platform-independent components, this result suggests that substantial coherence improvements can be achieved in a wide selection of quantum architectures.

Room-Temperature Quantum Emitter in Aluminum Nitride

A device that is able to produce single photons is a fundamental building block for a number of quantum technologies. Significant progress has been made in engineering quantum emission in the solid state, for instance, using semiconductor quantum dots as well as defect sites in bulk and two-dimensional materials. Here we report the discovery of a room-temperature quantum emitter embedded deep within the band gap of aluminum nitride. Using spectral, polarization, and photon-counting time-resolved measurements we demonstrate bright (>105 counts s-1), pure (g (2)(0) < 0.2), and polarized room-temperature quantum light emission from color centers in this commercially important semiconductor.

Theoretical polarization of zero phonon lines in point defects

In quantum technologies, point defects in semiconductors are becoming more significant. Understanding the frequency, intensity, and polarization of the zero phonon line is important. The last two properties are the subject of this paper. I present a method for calculating these properties and show the importance of using wave functions from both the ground and excited state. The validity of this method is demonstrated on the divacancy in 4H-SiC. Here, the calculated polarization and radiative lifetimes are in excellent agreement with experimental measurements. In general, this method can help to identify point defects and estimate suitable applications.

Dislocations as Single Photon Sources in Two-Dimensional Semiconductors

Single photon sources hold great promise in quantum information technologies and are often materialized by single atoms, quantum dots, and point defects in dielectric materials. Yet, these entities are vulnerable to annealing and chemical passivation, ultimately influencing the stability of photonic devices. Here, we show that topologically stable dislocations in transition metal dichalcogenide monolayers can act as single photon sources, as supported by calculated defect levels, diploe matrix elements for transition, and excitation lifetimes with first-principles. The emission from dislocations can range from 0.48 to 1.29 eV by varying their structure, charge state, and chemical makeup in contrast to the visible range provided by previously reported sources. Since recent experiments have controllably created dislocations in monolayer materials, these results open the door to utilizing robustly stable defects for quantum computing.

Coherent Control of Nitrogen-Vacancy Center Spins in Silicon Carbide at Room Temperature

Solid-state color centers with manipulatable spin qubits and telecom-ranged fluorescence are ideal platforms for quantum communications and distributed quantum computations. In this work, we coherently control the nitrogen-vacancy (NV) center spins in silicon carbide at room temperature, in which telecom-wavelength emission is detected. We increase the NV concentration sixfold through optimization of implantation conditions. Hence, coherent control of NV center spins is achieved at room temperature, and the coherence time T_{2} can be reached to around 17.1  μs. Furthermore, an investigation of fluorescence properties of single NV centers shows that they are room-temperature photostable single-photon sources at telecom range. Taking advantage of technologically mature materials, the experiment demonstrates that the NV centers in silicon carbide are promising platforms for large-scale integrated quantum photonics and long-distance quantum networks.

Coherent electrical control of a single high-spin nucleus in silicon

Nuclear spins are highly coherent quantum objects. In large ensembles, their control and detection via magnetic resonance is widely exploited, for example, in chemistry, medicine, materials science and mining. Nuclear spins also featured in early proposals for solid-state quantum computers¹ and demonstrations of quantum search² and factoring³ algorithms. Scaling up such concepts requires controlling individual nuclei, which can be detected when coupled to an electron^4-6. However, the need to address the nuclei via oscillating magnetic fields complicates their integration in multi-spin nanoscale devices, because the field cannot be localized or screened. Control via electric fields would resolve this problem, but previous methods^7-9 relied on transducing electric signals into magnetic fields via the electron-nuclear hyperfine interaction, which severely affects nuclear coherence. Here we demonstrate the coherent quantum control of a single ¹²³Sb (spin-7/2) nucleus using localized electric fields produced within a silicon nanoelectronic device. The method exploits an idea proposed in 1961¹⁰ but not previously realized experimentally with a single nucleus. Our results are quantitatively supported by a microscopic theoretical model that reveals how the purely electrical modulation of the nuclear electric quadrupole interaction results in coherent nuclear spin transitions that are uniquely addressable owing to lattice strain. The spin dephasing time, 0.1 seconds, is orders of magnitude longer than those obtained by methods that require a coupled electron spin to achieve electrical driving. These results show that high-spin quadrupolar nuclei could be deployed as chaotic models, strain sensors and hybrid spin-mechanical quantum systems using all-electrical controls. Integrating electrically controllable nuclei with quantum dots^11,12 could pave the way to scalable, nuclear- and electron-spin-based quantum computers in silicon that operate without the need for oscillating magnetic fields.

Room-Temperature Defect Qubits in Ultrasmall Nanocrystals

There is an urgent quest for room-temperature qubits in nanometer-sized, ultrasmall nanocrystals for quantum biosensing, hyperpolarization of biomolecules, and quantum information processing. Thus far, the preparation of such qubits at the nanoscale has remained futile. Here, we present a synthesis method that avoids any interaction of the solid with high-energy particles and uses self-propagated high-temperature synthesis with a subsequent electrochemical method, the no-photon exciton generation chemistry to produce room-temperature qubits in ultrasmall nanocrystals of sizes down to 3 nm with high yield. We first create the host silicon carbide (SiC) crystallites by high-temperature synthesis and then apply wet chemical etching, which results in ultrasmall SiC nanocrystals and facilitates the creation of thermally stable defect qubits in the material. We demonstrate room-temperature optically detected magnetic resonance signal of divacancy qubits with 3.5% contrast from these nanoparticles with emission wavelengths falling in the second biological window (1000-1380 nm). These results constitute the formation of nonperturbative bioagents for quantum sensing and efficient hyperpolarization.

Stark Tuning of the Silicon Vacancy in Silicon Carbide

We present a versatile scheme dedicated to exerting strong electric fields up to 0.5 MV/cm on color centers in hexagonal silicon carbide, employing transparent epitaxial graphene electrodes. In both the axial and basal direction equally strong electric fields can be selectively controlled. Investigating the silicon vacancy (V_Si) in ensemble photoluminescence experiments, we report Stark splitting of the V1' line of 3 meV by a basal electrical field and a Stark shift of the V1 line of 1 meV in an axial electric field. The spectral fine-tuning of the V_Si, being an important candidate for realizing quantum networks, paves the way for truly indistinguishable single-photon sources.

Electrically driven optical interferometry with spins in silicon carbide

Interfacing solid-state defect electron spins to other quantum systems is an ongoing challenge. The ground-state spin's weak coupling to its environment not only bestows excellent coherence properties but also limits desired drive fields. The excited-state orbitals of these electrons, however, can exhibit stronger coupling to phononic and electric fields. Here, we demonstrate electrically driven coherent quantum interference in the optical transition of single, basally oriented divacancies in commercially available 4H silicon carbide. By applying microwave frequency electric fields, we coherently drive the divacancy's excited-state orbitals and induce Landau-Zener-Stückelberg interference fringes in the resonant optical absorption spectrum. In addition, we find remarkably coherent optical and spin subsystems enabled by the basal divacancy's symmetry. These properties establish divacancies as strong candidates for quantum communication and hybrid system applications, where simultaneous control over optical and spin degrees of freedom is paramount.

Correlating dynamic strain and photoluminescence of solid-state defects with stroboscopic x-ray diffraction microscopy

Control of local lattice perturbations near optically-active defects in semiconductors is a key step to harnessing the potential of solid-state qubits for quantum information science and nanoscale sensing. We report the development of a stroboscopic scanning X-ray diffraction microscopy approach for real-space imaging of dynamic strain used in correlation with microscopic photoluminescence measurements. We demonstrate this technique in 4H-SiC, which hosts long-lifetime room temperature vacancy spin defects. Using nano-focused X-ray photon pulses synchronized to a surface acoustic wave launcher, we achieve an effective time resolution of ~100 ps at a 25 nm spatial resolution to map micro-radian dynamic lattice curvatures. The acoustically induced lattice distortions near an engineered scattering structure are correlated with enhanced photoluminescence responses of optically-active SiC quantum defects driven by local piezoelectric effects. These results demonstrate a unique route for directly imaging local strain in nanomechanical structures and quantifying dynamic structure-function relationships in materials under realistic operating conditions.

Dynamic strain in silicon carbide can tune point defect properties and coherently control their electron spins. Here the authors fabricate Gaussian-shaped surface acoustic wave transducers, use stroboscopic x-ray imaging to measure lattice dynamics, and observe its effects on defect photoluminescence.

Microscopic Imaging of the Stress Tensor in Diamond Using in Situ Quantum Sensors

The precise measurement of mechanical stress at the nanoscale is of fundamental and technological importance. In principle, all six independent variables of the stress tensor, which describe the direction and magnitude of compression/tension and shear stress in a solid, can be exploited to tune or enhance the properties of materials and devices. However, existing techniques to probe the local stress are generally incapable of measuring the entire stress tensor. Here, we make use of an ensemble of atomic-sized in situ strain sensors in diamond (nitrogen-vacancy defects) to achieve spatial mapping of the full stress tensor, with a submicrometer spatial resolution and a sensitivity of the order of 1 MPa (10 MPa) for the shear (axial) stress components. To illustrate the effectiveness and versatility of the technique, we apply it to a broad range of experimental situations, including mapping the stress induced by localized implantation damage, nanoindents, and scratches. In addition, we observe surprisingly large stress contributions from functional electronic devices fabricated on the diamond and also demonstrate sensitivity to deformations of materials in contact with the diamond. Our technique could enable in situ measurements of the mechanical response of diamond nanostructures under various stimuli, with potential applications in strain engineering for diamond-based quantum technologies and in nanomechanical sensing for on-chip mass spectroscopy.

Excitation and coherent control of spin qudit modes in silicon carbide at room temperature

One of the challenges in the field of quantum sensing and information processing is to selectively address and coherently manipulate highly homogeneous qubits subject to external perturbations. Here, we present room-temperature coherent control of high-dimensional quantum bits, the so-called qudits, associated with vacancy-related spins in silicon carbide enriched with nuclear spin-free isotopes. In addition to the excitation of a spectrally narrow qudit mode at the pump frequency, several other modes are excited in the electron spin resonance spectra whose relative positions depend on the external magnetic field. We develop a theory of multipole spin dynamics and demonstrate selective quantum control of homogeneous spin packets with sub-MHz spectral resolution. Furthermore, we perform two-frequency Ramsey interferometry to demonstrate absolute dc magnetometry, which is immune to thermal noise and strain inhomogeneity.

High-dimensional quantum bits advance the application of quantum sensing and information processing technologies but suffer from the low spectral selectivity and working temperature. Here the authors present the selective excitation and control of spin qudits modes based on an ensemble of silicon vacancy defects in silicon carbide at room temperature.

Spin-Mechanical Coupling of an InAs Quantum Dot Embedded in a Mechanical Resonator

We demonstrate strain-induced coupling between a hole spin in a quantum dot and mechanical motion of a cantilever. The optical transitions of quantum dots integrated into GaAs mechanical resonators are measured synchronously with the motion of the driven resonators. In a Voigt magnetic field, both electron and hole spin splittings are measured, showing negligible change for the electron spin but a large change for the hole spin of up to 36%. This large effect is attributed to the stronger spin orbit interaction of holes compared to electrons.

Electrometry by optical charge conversion of deep defects in 4H-SiC

Electric field sensing is an important tool in metrology and characterization applications. Here we show that photoluminescent defects in silicon carbide, such as divacancies and silicon vacancies, can provide local information of radio-frequency electric fields. Using all-optical excitation, the charge state of the defect is controlled, measured, and shown to be affected by this electric field. This sensing technique enables spatial 3D mapping as well as spectral resolution of the electric field. By taking advantage of the piezoelectricity in silicon carbide, the technique also provides similar information on local radio-frequency strain waves. This method is expected to be broadly applicable to other materials and of interest for high-power electronics and high-frequency microelectromechanical systems.

Optically active point defects in various host materials, such as diamond and silicon carbide (SiC), have shown significant promise as local sensors of magnetic fields, electric fields, strain, and temperature. Modern sensing techniques take advantage of the relaxation and coherence times of the spin state within these defects. Here we show that the defect charge state can also be used to sense the environment, in particular high-frequency (megahertz to gigahertz) electric fields, complementing established spin-based techniques. This is enabled by optical charge conversion of the defects between their photoluminescent and dark charge states, with conversion rate dependent on the electric field (energy density). The technique provides an all-optical high-frequency electrometer which is tested in 4H-SiC for both ensembles of divacancies and silicon vacancies, from cryogenic to room temperature, and with a measured sensitivity of 41±8 (V/cm)2/Hz. Finally, due to the piezoelectric character of SiC, we obtain spatial 3D maps of surface acoustic wave modes in a mechanical resonator.

Deep level defects in 4H-SiC introduced by ion implantation: the role of single ion regime

We characterized intrinsic deep level defects created in ion collision cascades which were produced by patterned implantation of single accelerated 2.0 MeV He and 600 keV H ions into n-type 4H-SiC epitaxial layers using a fast-scanning reduced-rate ion microbeam. The initial deep level transient spectroscopy measurement performed on as-grown material in the temperature range 150-700 K revealed the presence of only two electron traps, Z _1/2 (0.64 eV) and EH_6/7 (1.84 eV) assigned to the two different charge state transitions of the isolated carbon vacancy, V _C (=/0) and (0/+). C-V measurements of as-implanted samples revealed the increasing free carrier removal with larger ion fluence values, in particular at depth corresponding to a vicinity of the end of an ion range. The first DLTS measurement of as-implanted samples revealed formation of additional deep level defects labelled as ET1 (0.35 eV), ET2 (0.65 eV) and EH3 (1.06 eV) which were clearly distinguished from the presence of isolated carbon vacancies (Z _1/2 and EH_6/7 defects) in increased concentrations after implantations either by He or H ions. Repeated C-V measurements showed that a partial net free-carrier recovery occurred in as-implanted samples upon the low-temperature annealing following the first DLTS measurement. The second DLTS measurement revealed the almost complete removal of ET2 defect and the partial removal of EH3 defect, while the concentrations of Z _1/2 and EH_6/7 defects increased, due to the low temperature annealing up to 700 K accomplished during the first temperature scan. We concluded that the ET2 and EH3 defects: (i) act as majority carrier removal traps, (ii) exhibit a low thermal stability and (iii) can be related to the simple point-like defects introduced by light ion implantation, namely interstitials and/or complex of interstitials and vacancies in both carbon and silicon sub-lattices.

A review on single photon sources in silicon carbide

This paper summarizes key findings in single-photon generation from deep level defects in silicon carbide (SiC) and highlights the significance of these individually addressable centers for emerging quantum applications. Single photon emission from various defect centers in both bulk and nanostructured SiC are discussed as well as their formation and possible integration into optical and electrical devices. The related measurement protocols, the building blocks of quantum communication and computation network architectures in solid state systems, are also summarized. This includes experimental methodologies developed for spin control of different paramagnetic defects, including the measurement of spin coherence times. Well established doping, and micro- and nanofabrication procedures for SiC may allow the quantum properties of paramagnetic defects to be electrically and mechanically controlled efficiently. The integration of single defects into SiC devices is crucial for applications in quantum technologies and we will review progress in this direction.

Direct Nanoscale Sensing of the Internal Electric Field in Operating Semiconductor Devices Using Single Electron Spins

The electric field inside semiconductor devices is a key physical parameter that determines the properties of the devices. However, techniques based on scanning probe microscopy are limited to sensing at the surface only. Here, we demonstrate the direct sensing of the internal electric field in diamond power devices using single nitrogen-vacancy (NV) centers. The NV center embedded inside the device acts as a nanoscale electric field sensor. We fabricated vertical diamond p-i-n diodes containing the single NV centers. By performing optically detected magnetic resonance measurements under reverse-biased conditions with an applied voltage of up to 150 V, we found a large splitting in the magnetic resonance frequencies. This indicated that the NV center senses the transverse electric field in the space-charge region formed in the i-layer. The experimentally obtained electric field values are in good agreement with those calculated by a device simulator. Furthermore, we demonstrate the sensing of the electric field in different directions by utilizing NV centers with different N-V axes. This direct and quantitative sensing method using an electron spin in a wide-band-gap material provides a way to monitor the electric field in operating semiconductor devices.

High-Fidelity Bidirectional Nuclear Qubit Initialization in SiC

Dynamic nuclear polarization (DNP) is an attractive method for initializing nuclear spins that are strongly coupled to optically active electron spins because it functions at room temperature and does not require strong magnetic fields. In this Letter, we theoretically demonstrate that DNP, with near-unity polarization efficiency, can be generally realized in weakly coupled electron spin-nuclear spin systems. Furthermore, we theoretically and experimentally show that the nuclear spin polarization can be reversed by magnetic field variations as small as 0.8 Gauss. This mechanism offers new avenues for DNP-based sensors and radio-frequency free control of nuclear qubits.

Quantum decoherence dynamics of divacancy spins in silicon carbide

Long coherence times are key to the performance of quantum bits (qubits). Here, we experimentally and theoretically show that the Hahn-echo coherence time of electron spins associated with divacancy defects in 4H-SiC reaches 1.3 ms, one of the longest Hahn-echo coherence times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we find that two factors determine the unusually robust coherence. First, in the presence of moderate magnetic fields (30 mT and above), the ²⁹Si and ¹³C paramagnetic nuclear spin baths are decoupled. In addition, because SiC is a binary crystal, homo-nuclear spin pairs are both diluted and forbidden from forming strongly coupled, nearest-neighbour spin pairs. Longer neighbour distances result in fewer nuclear spin flip-flops, a less fluctuating intra-crystalline magnetic environment, and thus a longer coherence time. Our results point to polyatomic crystals as promising hosts for coherent qubits in the solid state.

Electron-vibration coupling induced renormalization in the photoemission spectrum of diamondoids

The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born–Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron–vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron–vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born–Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn–Teller effect.

The electron–vibration coupling is essential to describe the photoelectron properties of molecules. Here, the authors show theoretically and experimentally that the electron–vibration coupling is very large in diamonoids, and link the deduced vibronic states to the well-known Jahn–Teller effect.

Modulation of nitrogen vacancy charge state and fluorescence in nanodiamonds using electrochemical potential

The negatively charged nitrogen vacancy (NV(-)) center in diamond has attracted strong interest for a wide range of sensing and quantum information processing applications. To this end, recent work has focused on controlling the NV charge state, whose stability strongly depends on its electrostatic environment. Here, we demonstrate that the charge state and fluorescence dynamics of single NV centers in nanodiamonds with different surface terminations can be controlled by an externally applied potential difference in an electrochemical cell. The voltage dependence of the NV charge state can be used to stabilize the NV(-) state for spin-based sensing protocols and provides a method of charge state-dependent fluorescence sensing of electrochemical potentials. We detect clear NV fluorescence modulation for voltage changes down to 100 mV, with a single NV and down to 20 mV with multiple NV centers in a wide-field imaging mode. These results suggest that NV centers in nanodiamonds could enable parallel optical detection of biologically relevant electrochemical potentials.

Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.

Optically Addressable Silicon Vacancy-Related Spin Centers in Rhombic Silicon Carbide with High Breakdown Characteristics and ENDOR Evidence of Their Structure

We discovered a family of uniaxially oriented silicon vacancy-related centers with S=3/2 in a rhombic 15R-SiC crystalline matrix. We demonstrate that these centers exhibit unique characteristics such as optical spin alignment up to the temperatures of 250°C. Thus, the range of robust optically addressable vacancy-related spin centers is extended to the wide class of rhombic SiC polytypes. To use these centers for quantum applications it is essential to know their structure. Using high frequency electron nuclear double resonance, we show that the centers are formed by negatively charged silicon vacancies V_{Si}^{-} in the paramagnetic state with S=3/2 that is noncovalently bonded to the neutral carbon vacancy V_{C}^{0} in the nonparamagnetic state, located on the adjacent site along the SiC symmetry c axis.

Quantum entanglement at ambient conditions in a macroscopic solid-state spin ensemble

Entanglement is a key resource for quantum computers, quantum-communication networks, and high-precision sensors. Macroscopic spin ensembles have been historically important in the development of quantum algorithms for these prospective technologies and remain strong candidates for implementing them today. This strength derives from their long-lived quantum coherence, strong signal, and ability to couple collectively to external degrees of freedom. Nonetheless, preparing ensembles of genuinely entangled spin states has required high magnetic fields and cryogenic temperatures or photochemical reactions. We demonstrate that entanglement can be realized in solid-state spin ensembles at ambient conditions. We use hybrid registers comprising of electron-nuclear spin pairs that are localized at color-center defects in a commercial SiC wafer. We optically initialize 10(3) identical registers in a 40-μm(3) volume (with [Formula: see text] fidelity) and deterministically prepare them into the maximally entangled Bell states (with 0.88 ± 0.07 fidelity). To verify entanglement, we develop a register-specific quantum-state tomography protocol. The entanglement of a macroscopic solid-state spin ensemble at ambient conditions represents an important step toward practical quantum technology.

Fabrication of High-Q Nanobeam Photonic Crystals in Epitaxially Grown 4H-SiC

Silicon carbide (SiC) is an intriguing material due to the presence of spin-active point defects in several polytypes, including 4H-SiC. For many quantum information and sensing applications involving such point defects, it is important to couple their emission to high quality optical cavities. Here we present the fabrication of 1D nanobeam photonic crystal cavities (PCC) in 4H-SiC using a dopant-selective etch to undercut a homoepitaxially grown epilayer of p-type 4H-SiC. These are the first PCCs demonstrated in 4H-SiC and show high quality factors (Q) of up to ∼7000 as well as low modal volumes of <0.5 (λ/n)(3). We take advantage of the high device yield of this fabrication method to characterize hundreds of devices and determine which PCC geometries are optimal. Additionally, we demonstrate two methods to tune the resonant wavelengths of the PCCs over 5 nm without significant degradation of the Q. Lastly, we characterize nanobeam PCCs coupled to luminescence from silicon vacancy point defects (V1, V2) in 4H-SiC. The fundamental modes of two such PCCs are tuned into spectral overlap with the zero phonon line (ZPL) of the V2 center, resulting in an intensity increase of up to 3-fold. These results are important steps on the path to developing 4H-SiC as a platform for quantum information and sensing.

Dominant luminescence is not due to quantum confinement in molecular-sized silicon carbide nanocrystals

Molecular-sized colloid silicon carbide (SiC) nanoparticles are very promising candidates to realize bioinert non-perturbative fluorescent nanoparticles for in vivo bioimaging. Furthermore, SiC nanoparticles with engineered vacancy-related emission centres may realize magneto-optical probes operating at nanoscale resolution. Understanding the nature of molecular-sized SiC nanoparticle emission is essential for further applications. Here we report an efficient and simple method to produce a relatively narrow size distribution of water soluble molecular-sized SiC nanoparticles. The tight control of their size distribution makes it possible to demonstrate a switching mechanism in the luminescence correlated with particle size. We show that molecular-sized SiC nanoparticles of 1-3 nm show a relatively strong and broad surface related luminescence whilst the larger ones exhibit a relatively weak band edge and structural defect luminescence with no evidence of quantum confinement effect.